1. Field of the Invention
The invention relates to heat curable, reaction resin mixtures as well as to reaction resin molded materials prepared from them.
2. Description of Related Art
Epoxy resins, in particular acid-anhydride curable epoxy resins, play an important role in electrical engineering. Namely, they are used in many application areas as solvent free reaction resins, in particular for insulating purposes. Thus, epoxy resins are used for producing insulating components, for insulating electrical windings and for covering and encasing electronic components and modules as well as in layering materials. The reaction resin molded materials which are prepared from epoxy resins have comparatively good thermo-mechanical properties. The recent, ever-growing trend in electronic engineering towards greater efficiency per volumetric unit or towards miniaturization, however, requires an application range for epoxy resin molded materials which is constantly expanding both in the direction of higher as well as lower temperatures. This requires molded materials with even better thermo-mechanical properties, for example such as improved thermal shock resistance along with simultaneously increased dimensional stability under heat (glass transition temperature).
These requirements can be satisfied by resinous compositions consisting of polyfunctional epoxides (EP) and polyfunctional isocyanates (IC). In the presence of suitable curing catalysts, these types of compositions polymerize into reaction resin molded materials, dimensionally stable under heat, which mostly consist of oxazolidinone structures (OX) and isocyanurate structures (ICR) (c.f. e.g.: DE-OS 25 43 386, DE-OS 33 23 153, DE-OS 36 00 764). By varying the initial molar ratio of the epoxide- and isocyanate functions, by the type of reaction accelerator, i.e., curing catalyst, and by selecting suitable curing temperatures, the concentration of OX- or ICR structures and thus the properties of the OX/ICR molded materials can be varied within broad limits. By using EP/IC resinous mixtures with a large surplus of isocyanate constituents, e.g. EP:IC&lt;0.2, molded materials are obtained which predominantly consist of ICR structures. Although these types of molded materials have very high glass transition temperatures (&gt;260.degree. C.), they exhibit relatively poor mechanical properties; e.g., a low impact resistance. In contrast, if EP/IC resins are used with an EP:IC molar ratio of 1 together with imidazoles as curing catalysts, then one obtains molded materials with a high oxazolidinone concentration (OX:ICR&gt;1) that have excellent mechanical properties at simultaneously high glass transition temperatures of approximately 200.degree. C. OX/ICR molded materials consisting predominantly of ICR structures are very well suited as winding insulations for electrical machines, which are dimensionally stable under heat and long-term temperature-resistant, and for the construction of first rate layering materials; whereas, due to their excellent mechanical properties and thermal shock resistance, filler-containing OX/ICR molded materials with an OX:ICR ratio of &gt;1 are very well suited for the production of insulating components as well as to the casting and encasing of electronic components.
To comply with national and international fire-protection regulations, polymer materials increasingly often must be inflammable or self-extinguishing in electrical engineering, in particular in electronics. For this purpose, the passing of one of the most stringent material testing standards is required, namely the flammability test according to Underwriter Laboratories Standard UL 94V with the rating V-0. During this test, five vertically clamped standard test pieces, respectively, are twice subjected to flame at the lower end for 10 seconds. The total of the ten postburning times, which cease at the point of extinguishment, must be &lt;50 seconds and no individual value is allowed to exceed 10 seconds. This requirement is difficult to satisfy in the case of thin wall thicknesses of 1.6 mm and less which is typical in electronics.
It has been known for a long time that nitrogenated heterocyclic structures such as oxazolidinone- and isocyanurate structures reduce the flammability of reaction resin molded materials. Thus, for example, OX/ICR molded materials with high dimensional stability under heat, excellent long-term temperature resistance and inflammable or self-extinguishing properties are known from the teachings of DE-OS 23 59 386; however, closer specifications are not indicated. Our own analyses have shown, however, that the flame-retarding properties of such reaction resin molded materials, even at a concentration of 65 to 70% of inorganic filling agents like quartz powder and dolomite, are not sufficient to result in a rating of UL 94V-0 at wall thicknesses of 1.6 mm.
Also known is the use of EP/IC resins to prepare foamed materials which have acceptable flameproofness. The preparation of polymers with oxazolidinone and carbodiimide structures, which have an LOI-value (Limiting Oxygen Index) up to approximately 27, is described in DE-OS 25 51 631. There is a report in "J. Cell. Plastics", Vol. 13 (1977), pp 399 to 403, about testing the inflammability of oxazolidinone-modified isocyanurate foam: LOI values of 26 to 28 are thereby indicated for OX/ICR foamed materials having an 8 to 10 times surplus of isocyanate. The LOI values for materials known to be inflammable with the rating V-0, such polysulfones (LOI=30), polyvinyl chloride (LOI=42), polyvinylidene chloride (LOI=60) and polytetra fluoroethylene (LOI=95), however, are considerably higher (c.f.: D.W. v. Krevelen "Properties of Polymers", Elsevier Scientific Publishing Comp., Amsterdam, Oxford, N. Y., 1976, pp 526 fol.).
All currently known OX/ICR molded materials which are prepared from EP/IC resins have the disadvantage of not inherently being sufficiently inflammable. That is, they do not satisfy the indispensable requirement in electrical engineering--and particularly in electronics--of passing the flammability test according to UL 94V with the rating V-0 even at layer thicknesses of 1.6 mm and less. There are many basic possible ways known for improving the inflammability of reaction resin molded materials. In epoxy resin molded materials, often considerable amounts of nucleus-brominated aromatic epoxy resin constituents or halogenated additives and high concentrations of antimony trioxide are used to regulate inflammability. The problem with these compounds is that on the one hand, they are extremely effective as flame retardant agents, but on the other hand, they also have very dangerous properties. Thus, antimony trioxide is on the list of carcinogenic chemicals, and, during thermal decomposition, aromatic bromine compounds split off not only bromine radicals and hydrogen bromide, which cause pronounced corrosion, but also, in the case of decomposition in the presence of oxygen, the highly-brominated aromatics in particular can form the highly toxic polybromodibenzofuranes and polybromodibenzodioxins. The disposal of bromine-containing waste materials and toxic waste also presents considerable difficulties.
Furthermore, the addition of the following filling agents, or the partial substitution of customary inorganic filling agents by these types of materials, has already been suggested: filling agents with a quenching gas effect, such as hydrated aluminum oxides (c.f.: "J. Fire and Flammability", Vol. 3 (1972), pp 51 fol.), alkaline aluminum carbonates (c.f.: "Plast. Engng.", Vol. 32 (1976), pp 41 fol.) and magnesium hydroxides (EP-OS 0 243 201); or vitrifying filling agents such as borates (c.f. "Modern Plastics", Vol. 47 (1970), pp 140 fol.) and phosphates (U.S. Pat. No. 2,766,139 and U.S. Pat. No. 3,398,019). However, all these filling agents have the disadvantage that they in part considerably deteriorate the mechanical, chemical and electrical properties of the molded materials. Moreover, they sometimes drastically deteriorate the processing properties of the cast resins (abrasiveness, alkalinity). Furthermore, in the case of OX/ICR molded materials, the splitting off of the quenching gas is already to be feared during the preparation of the molded material due to the requisite high post-curing temperatures of up to 200.degree. C.
Organic phosphorus compounds such as phosphoric acid esters, phosphonic acid esters and phosphines have also already been suggested as flame-retardant additives for epoxy resins (c.f.: W. C. Kuryla and A. J. Papa "Flame Retardancy of Polymeric Materials", Vol. 1, pp 24 to 38 and 52 to 61, Marcel Dekker Inc., New York, 1973). Since these compounds are known for their "plasticizing" properties and are used to a large extent worldwide as plasticizers for polymers (c.f. GB-PS 10 794), this alternative, too, is not very promising with regard to the heat resistance required of molded materials.
Accordingly, it is an object of the invention to provide heat curable reaction resin mixtures (based on EP/IC resins), which have good processing properties due to a low viscosity at the processing temperature and a long shelf life.
It is a further object of the invention to provide heat curable reaction resin mixtures which can be economically converted by conventional methods for processing cast resins into reaction resin molded materials which are inherently (i.e., without the addition of halogen compounds or antimony trioxide) inflammable and thus are rated V-0 according to UL 94V without having the high dimensional stability under heat as well as the good electrical and mechanical properties, which OX/ICR molded materials have, being negatively influenced.